(1) Field of the Invention
The present invention relates to a process for the preparation of a naphthalene-2,6-dicarboxylic acid dialkali metal salt. A naphthalene-2,6-dicarboxylic acid dialkali metal salt is easily converted to the corresponding carboxylic acid, tht is, naphthalene-2,6-dicarboxylic acid, by acidification. Recently, this naphthalene-2,6-dicarboxylic acid has attracted attention as the acid component of the starting materials for the preparation of a heat-resistant polyester, and is an industrially valuable compound.
(2) Description of the Related Art
The so-called Henkel process is known as one process for the preparation of a naphthalene-2,6-dicarboxylic acid dialkali metal salt. According to this process, an alkali metal salt of naphthalene-monocarboxylic acid (that is, naphthoic acid), a dialkali metal salt of naphthalene-dicarboxylic acid or a mixture thereof heated under pressure with carbon dioxide gas at a temperature higher than about 350.degree. C., in the presence of a catalyst such as an oxide of cadmium, zinc or mercury or a halide, sulfate, carbonate or carboxylate thereof. From an alkali metal naphthoate, a naphthalene-2,6-dicarboxylic acid dialkali metal salt and naphthalene are formed by disproportionation reaction, and from a naphthalene-dicarboxylic acid dialkali metal salt, a naphthalene-2,6-dicarboxylic acid dialkali metal salt is selectively formed by rearrangement reaction [see (a) B. Raecke, Angew. Chem., 70, 1 (1958), (b) B. Raecke et al, Org. Syn. Coll. Vol. 5, 813 (1973), (c) Yamashita et al, Journal of Synthetic Organic Chemistry, Japan, 20, 501 (1962), (d) E. McNelis, J. Org. Chem., 30, 1209 (1965), (e) U.S. Pat. No. 2,823,231, and (f) Japanese Unexamined Patent Publication No. 51-10224].
All of the naphthoic acid alkali metal salt and naphthalene-dicarboxylic acid dialkali metal salt used as the starting material in the Henkel process and the naphthalene-2,6-dicarboxylic acid dialkali metal salt as the product of the Henkel process have a melting point higher than 400.degree. C. These alkali metal salts are soluble in protonic solvents such as water and alcohols. However, at a reaction temperature higher than 350.degree. C., as adopted in the Henkel process, a protonic solvent reacts with and decomposes the alkali metal salts, and therefore, the protonic solvent cannot be used as the reaction medium. Accordingly, where a naphthalene-2,6-dicarboxylic acid dialkali metal salt is prepared according to the Henkel process, a method has been adopted in which a powdery mixture of the starting material and catalyst is heated and reacted in the absence of a solvent under pressure with carbon dioxide gas and a solid product is obtained.
Since both the starting material and product are solid, this method is disadvantageous in comparison with the method using liquids in that the operations of charging the starting material in a reaction vessel and withdrawing the reaction product from the reaction vessel become complicated. Furthermore, we found that since the reaction is carried out in the absence of a solvet in the conventional method, the reproducibility of the reaction is not satisfactory and the yield and selectivity of the intended product are low.
More specifically, the reaction is ordinarily effected by heating the reaction vessel, and since the starting material and product are solids having a high melting point and no solvent is present, heat transfer in the reaction mixture is slow and heterogeneous. Accordingly, a certain portion of the reaction mixture is excessively heated and a large amount of carbonized matter is formed, while in another portion of the reaction mixture, heating is insufficient and the reaction is incomplete, with the result that a large amount of the starting material still remains or formation of isomers other than the intended naphthalene-2,6-dicarboxylic acid dialkali metal salt as a by-product becomes prominent. Accordingly, the yield of and selectivity to the intended naphthalene-2,6-dicarboxylic acid dialkali metal salt are reduced and the reproducibility of the reaction is not satisfactory.